create a folder in a specific path using Blender python
You have following options. I use the file name myfile.xxx as placeholder for whatever file shall be opened.

Open within the folder where blender is installed:

    fout = open("myfile.xxx") 


However you might run into permission issues when the installation folder is write protected.

Open in the user Application folder (Appdata)

    import bpy
    import os
    userdir = bpy.utils.resource_path('USER') # see note
    mypath  = os.path.join(userdir,"myfile.xxx")

    fout    = open(mypath)


Note: Please check also bpy.utils.user_resource(...)

Open in the same folder where the current blend file is located

    import bpy
    import os
    blenddir = py.path.abspath('//')
    mypath   = os.path.join(userdir, "myfile.xxx")

    fout     = open(mypath)


Note: If the current session has not yet been saved to a blend file, then blenddir is returned as empty string! You might need to test for this in your script.

Open in addon folder

    import bpy
    import os
    import myaddon
    myhome = os.path.dirname(myaddon.__file__)
    mypath = os.path.join(myhome, "myfile.xxx")

    fout     = open(mypath)


Hint: When you have installed the Addon by using the Addon Tab in User Preferences, then myhome points to Blender's scripts/addons/myaddon folder in the Appdata directory.

Open in Preset folder (where to store addon presets)

    import bpy
    import os
    import myaddon
    presets   = bpy.utils.user_resource('SCRIPTS', "presets")
    mypresets = os.path.join(presets, myaddon.__name__)
    if not os.path.exists(mypresets):
        os.makedirs(mypresets)    
    mypath = os.path.join(mypresets, "myfile.xxx")

    fout     = open(mypath)